There has been a known air pollution control system for treating flue gas discharged when fuel is burnt by a combustion engine such as a boiler installed in a thermal power generation facility, and the like. For example, as described in Patent Literature 1, the air pollution control system includes a denitration device that removes nitrogen oxides from flue gas from a boiler, an air heater that recovers heat of the flue gas passing through the denitration device, a heat recovery device that further recovers heat of the flue gas and uses the recovered heat to increase a temperature of the flue gas at a stack entrance, a dust collector that removes dust in the flue gas after the heat recovery, and a desulfurization device that removes sulfur oxides in the flue gas after the dust removing. A system in which the dust collector is installed in front of the heat recovery device may be used in some cases. A wet type desulfurization device that removes sulfur oxides in the flue gas by allowing a limestone absorbent, and the like to come into gas-liquid contact with the flue gas is generally used as the desulfurization device. The device described in Patent Literature 1 supplies charged solid particles to a flue gas duct on a downstream side of the dust collector.